Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
This invention relates to a process for the
electrolytic oxidation of sodiumdialkyldithiocarbamates to
tetraalkylthiuram disulfides.
Tetraalkylthiuram disulfides are commercially
important in industry and agriculture as, for example,
vulcanization accelerators, fugicides, and seed treating
agents. The usual industrial method of making these com- -
pounds involves oxidation of dialkyldithiocarbamates with
chlorine. Because of overoxid~tion, which cannot be avoided,
the yield of the chlorine oxidation process does not exceed
about 88~. The overoxidation products, large quantities of
sodium chloride, and a small amount of the thiuram disulfide
are removed in the waste stream.
Electrolytic oxidation of dialkyldithiocarbamates
to tetraalkylthiuram disulfides theoretically appears to be
a much better alternative since it should be capable of ~-
producing purer product in a higher yield and would not
present as environmentally serious waste disposal problems
; 20 as does the chlorine oxidation method. The electrochemical
reaction has been attempted in the past but without much
success. Thus, U.S.S.R. Patent 53,766 (1938) discloses a
process for the continuous electrolysis of sodium dimethyl~
dithiocarbamate using a scraped, rotating nickel anode.
A thin sheet of asbestos is inserted between the anode and
the cathode, but its purpose is not explained in the patent. ~-~
The necessity of using a rotating anode is a serious short-
coming of this process because it usually is difficult to
maintain good chemical contact between a rotating
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electrode and the source of electrical current. Apparently
the combination of rot~ting anode and a scraper, which
removes the product, avoids excessive build-up of the pro-
duct on the anode.
U.S. 2,385,410 (1945) describes an electrolytic
process using alternating current to avoid deposition of
the product on the electrodes. Direct current electrolysis
re~uiring scraped electrodes is said to be awkward and
inconvenient. The product appears, however, to have been
obtained in low yield and in a state of questionable purity.
Because, according to the patentee, a neutral medium is
preferred, pH control is important. Acid is added gradually
to the cell to neutralize caustic generated during the ~;
electrolysis.
It can be seen that the electrochemical production ~
oE tetraalkylthiuram disulfides from dialkyldithiocarbamates ;
has no~ lived up to the expectations, and that an improved ~;
process would be very desirable.
SU~ARY OF THE INVENTION
The present invention provides an improved process
for a direct current electrolytic oxidation of dialkyldithio-
carbamates to tetraalkylthiuram disulfides, wherein the
electrolytic cell is divided into the cathode compartment and
the anode compartment separated from each other by a cationic
membrane capable of resisting migration of hydroxyl ions
under the electrolysis conditions. The only active anode
surfaces exposed to the anolyte are shiny platinum surfaces;
the anolyte is an alkali metal dialkyldithiocarbamate
solution; and the catholyte is a dilute alkali solution. ~;
It is preferred to operate the process of this
invention at an anode current clensity of a-t least 0.2 amp/cm2
at sodium dialkyldithiocarbamate concentration of 20-40 weight
percent, and at anolyte temperature of at least 60C.
THE DRAWING
The drawing schematically represents a complete
process flow sheet for a typical plant unit according to the ~-
present invention. ;
DETAILED DESCRIPTION OF THE INVENTION
The chemical reactions occurring in an electrolytic
cell according to this invention are represented by the ~ol-
lowing equations: S S S
Anode: 2 R2NCSNa ~ R2NCSSCNR2~ 2Na + 2e
Cathode: 2 Na + 2H2O ~ 2e ~ 2NaOH + H2
Because of the cationic membrane separating the
electrode compartments, sodium hydroxide formed at the cathode
cannot enter the anode compartment ancl increase the alkalinity
oE the anolyte. Because of this feature, the process of the ;~
present invention does not require neutralization of the ;~
anolyte which was necessary in the process of the above-
discussed U.S. 2,385,410.
Another problem which plagued prior attempts was
product build-up on the anode. It has now been unexpectedly
discovered that shiny platinum is the only active anode
material which is not subject to product build-up, especially
if the anolyte is agitated. It is not necessary that the
entire anode be ~ade of shining platinum, such as foil or wire,
but it can also be made by rolling a layer of platinum on a
suitable substrate, such as, for example, titanium, tantalum,
and columbium. These metals are passive in contact with the
anolyte and will not cause product accumulation.
The cathode may be made o-F any suitable material.
The most commonly used cathode material is mild steel. Other
possible materials include, for example, stainless steel
and titanium. While precious metals such as platnum, gold,
iridium, or palladium, also are suitable cathode materials,
their high cost makes them impractical for this application.
With presently available cationic membranesj the
sodium hydroxide concentration in the cathode compartment
preferably should not be higher than about 17 weight percent.
Above this concentration, the cationic membrane would lose
the essential selectivity and would allowhydroxyl ions
into the anode compartment in amounts which would alter
the p~ and bring about the formation of undesirable by~
products. However, as membranes which still are selective
at high caustic concentrations become available, such
higher concentrated caustic can be used. The catholyte is
continuously diluted by water because each Na ion going
through the cationic membrane is accompanied by about
twelve water molecules. The number of molecules of water
that pass through the membrane ~or each Na ion depends
on the membrane used. Additional water may be added, if
desired, directly to the catholyte continuously or inter-
mittently. Excess catholyte usually will be drained.
While under the preferred conditions the anolyte
temperature is at least 60C, the catholyte temperature
may be lower or higher. Usually, there will be a difference
of a few degrees between the electrolytes in both compartments.
The discovery that shiny platinum is the only
suitable active anode surface material is surprising
because other metals can be obtained in the same degree of
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surface smoothness and are a~ inert chemically, yet are un-
suitable. These include, for example, gold, nickel and
stainless steel. It is not certain whether material build-up
encountered with such materials in prior art processes is due
to the fact that impure, sticky product is formed which tends
to adhere to the anode surface; or, conversely, the produc~
which builds up on the anode is eventually decomposed in part
and thus is of inferior quality. The product obtained by the
process of the present invention is, however, white and has a
high melting point; it is a high purity material.
While the present disclosure is mainly concerned
with the electrolysis of sodium dialkyldithiocarbamates,
o-ther dial~yldithiocarbamate salts can be used in this process.
These would be especially potassium and lithium salts but may
also be other alkali metal, ammonium, and quarternary
ammonium salts.
The catonic membrane required in the process of
the present invention can be any commercially available,
organic or inorganic membrane, such as, for example, a
Nafion~ cationic membrane available from E.I. du Pont
de Nemours and Company, Wilmington, Delaware.
The preferred dialkyldithiocarbamate concentration
in the anode compartment provides maximum current efficiency.
A 30% solution has the highest conductivity. The conductivity
of solutions more dilute than 20% may be too low for practical
operation; above 40~, a slurry is formed and the conductivity
is ~uite low. In addition, outside the preferred concentra~
tion limits danger of overoxidation arises. The desired
current efficiency is at least about 90%. The "inefficient"
current may produce either innocuous products such as
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hydrogen and oxygen from electrolytic decomposition of
water or tetraalkyl-thiuram disulfide degradation products,
which should be avoided.
The process of this invention can be run with a
direct current of constant polarity, or the direction of
current may be periodically reversed for short time intervals.
In practice the current reversal will not normally be ;
required.
Referring now to the drawing, the process of the
present invention can be operated according to the flow
sheet therein.
A dialkylamine, carbon disulfide and recycled ;~
sodium hydroxide are combined to form sodium dialkyl-
dithiocarbamate in the "salt reactor" (1). To the product
from this reactor is added filtrate and wash water (2) from
the final product isolation steps so that unchan~ed dithio-
carbamate can be recovered. These streams are heated in an
evaporator (3) and enough water is evelporated to give a
feed stream (4) to the electrolysis cell anode compartment of ;
the desired dithiocarbamate concentration. Since impurities
built up in the recycle streams will be at the highest con~
centration in this stream, a purge (24) is provided here so
that impurity levels will equilibrate. The dithiocarbamate
solution is electrolyzed in the anode compartment (5) of the
electrolytic cell which is separated from the cathode compart~
ment (6) by a cationic membrane (7). The effluent from the
anode compartment (8) contains precipitated tetraalkylthiuram
product. Solids in this effluent stream are concentrated in a
settling tank (9) to give dialkyldithiocarbamate solution for ;; ;~
recycle (10) and a more concentrated slurry of product tetra-
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alkylthiuram disulfide tll). The slurry is filtered and
washed wi-th water in filter (12) to give a wet filter cake pro-
duct (13). The filtrate and wash wa-ter (2) are recycled as
described above. The wash water (14) is provided from water
storage tank (15). This tank is supplied by the water
evaporated from the evaporator (16)and needed make-up water
(17). This water supply also furnishes the make-up water
for the catholyte (18) which enters the cathode compartment
(6) along with recycled caustic solution (19). Effluent from
the cathode compartment (20) is degassed in liquid-gas
separator (21) to give by-product hydrogen (22) and caustic
for recycle as catholyte (19) and for use in the salt reactor
(23). The caustic solution recycled to the cathode compart-
ment contains at most 17~ by weight of sodium hydroxide.
This scheme makes a very neat process in which the
only outflows from the process are the wet cake product (13),
by-product hydrogen (2~) and a small liquid purge stream (24).
This process offers the following advantages:
(1) A white, high-purity thiuram product is
obtained electrochemically.
(2) Anode scraping devices are not needed so that
standard electrochemical processing equipment
can be used.
(3) The sodium hydroxide generated at the cathode
is of a high quality and can be recycled to
the reactor where the sodium dithiocarbamate
salt is formed.
This invention is now illustrated by the following
examples of certain repre~entative embodiments thereof, where
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q~s
all parts, proportions, and percentages are by weight
unless otherwise indicated.
EXAMPLE 1
A glass electrolysis cell with two 300 ml. compart
ments separated by a Nafion~ Type 427 cationic membrane was
fitted with two 10 cm2 electrodes made of 5 mil platinum foil.
To the anode compartment was added 300 ml of aqueous solution
containing 137 grams of sodium dimethyldithiocarbamate (40%
dithiocarbamate). The catholyte was 300 ml of 0.49_ sodium
hydroxide. A current of 3 amp was passed through the cell for
one hour while the anolyte and catholyte were magnetically
stirred. At the end of this time the anolyte was filtered,
and pure, white tetramethyl thiuram disulfide with a melting
point of 148.8C)C was recovered. Conversion of the sodium di- ;
methyldithiocarbamate was about 10%. Current efficiency was
88.5%. Product did not adhere to the anode during this opera-
tion. The temperature of the anolyte was measured as 64C to-
ward the end of the operation. By ma~erial balance, 95.5% of
the electrolyzed dithiocarbamate was accounted for as the
0 tetramethylthiuram product recovered.
EXAMPLE 2
This comparative experiment was carried out under
the same conditions as Example 1 except that a single 300 ml
beaker housed both electrodes. No membrane was used in the
cell. The beaker was charged with 300 ml of sodium dimethyl-
dithiocarbamate solution. A 3 amp current was passed through
the cell for one hour while the solution was magnetically ~ -
stirred. At the end of this time the solution was filtered to
give 2.4 grams of product when dry. This is equivalent to 2.1%
30 conversion of the dithiocarbamate present and a current effic- ; ,~
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iency of about 18%~
EXAMPLE 3
Conditions of Example 1 were reproduced except that
a 2.5 amp current was passed through the cell for four hours.
A pure white product (35.1 g) was obtained which had a melting
point of 145C. Current efficiency was 78.3%. Conversion of
sodium dimethyldithiocarbamate was about 25%. Thus good pro-
duct was produced in Examples 1 and 3 at high current effic-
iencies at 0.25 and 0.30 amp/cm current densities.
EXAMPLE 4
Conditions of Example 3 were repeated except that
the temperature was not allowed to rise to the usual 60-90C.
With an ice bath around the anolyte, the temperature was main-
tained at 20-28C. After a 2.5 amp current was passed for 4
hours, 14.55 g. of a yellow product was recovered by filtra-
tion and drying. Current efficiency was only 32.5%. This
shows the undesirability of operating the electrochemical cell
at a temperature well below the stated minimum temperature.
EXAMPLE 5
The same apparatus and electrolyte solutions as used
in Examplesl, 3 and 4 were used here. A lower current of 1
amp was passed for 4 hours. This resulted in 6.21 g. of impure
product with a melting point of 132C. Current efficiency was
only 34.7%. It appears desirable to operate at current den-
sities of 0.2 amp/cm or greater for satisfactory cell opera-
tion rather than at the lower current density (0.1 amp/cm2) of
this example. `
EXAMPLE 6
The same conditions as shown in Example 1 were used
30 here. A 3 amp current was passed for 2 hours giving 23.5 g. ~ ~
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of a white product. Anolyte temperature toward the end of
cell operation was about 76C. Current efficiency was
87.~.
EX~MPLE 7
The same apparatus and conditions used in Example
6 were used here except that only 27.4 g. of sodium dimethyl
dithiocarbamate were in the anolyte. Thus the solution was
only 8~ dithiocarbamate by weight rather than the 40
normally used. After a 3 amp current was passed through
the cell for 2 hrs, 5.9 g. of a yellow product were
10 recovered. Anolyte temperature had reached 90C. The ~'
current efficiency was only 21.9~.
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